1. Technical Field
This disclosure generally relates to the field of illumination devices and, more particularly, to regulation of electrical power applied to solid-state lighting in an illumination device.
2. Description of the Related Art
With increasing trend of energy conservation and for various other reasons, including replacement of gas-vapor lamps, solid-state lighting has become more and more popular as the source of illumination in a wide range of applications. As generally known, solid-state lighting refers to a type of lighting that emits light from a solid object, such as a block of semiconductor, rather than from a vacuum or gas tube as is the case in traditional lighting. Examples of solid-state lighting include light-emitting diodes (LEDs), organic light-emitting diodes (OLEDs), and polymer light-emitting diodes (PLEDs). Solid-state lighting as compared to traditional lighting generates visible light with reduced parasitic energy dissipation in the form of reduced heat generation. Further, solid-state lighting tends to have increased lifespan compared to traditional lighting. This is because, due to its solid-state nature, solid-state lighting provides for greater resistance to shock, vibration, and wear.
An LED illumination device is a type of solid-state lighting that utilizes LEDs as a source of illumination, and typically has clusters of LEDs in a suitable housing. The LEDs in an LED illumination device typically have very low dynamic resistance, with the same voltage drop for widely-varying currents. Thus, the LEDs cannot be connected directly to most power sources, such as the 120-volt alternating current (AC) mains commonly available in the U.S., without causing damages to the LEDs. LEDs typically conduct current in one direction, and require a current that does not exceed the maximum current rating of the LED.
Two methods have been typically used to limit the current that is applied to LEDs in an illumination device to a safe level. The first method uses an electronic switching ballast that converts the AC input voltage form the power mains into a direct current (DC) regulated current of an acceptable value. The second method uses a string of LEDs coupled in series where a voltage drop of the string equal to the input voltage at the current limit.
An electronic switching ballast that regulates current typically employs a switching current and a magnetic energy storage device such as a series inductor (e.g., in a buck regulator) or transformer (e.g., in a flyback regulator). Various different topologies have been developed in the attempt to obtain high conversion efficiencies. Still, a typical flyback or buck type regulator will have a conversion efficiency of only 60% to 90%, wasting 10% to 40% of the input power in the form of heat.
Electronic switching ballasts tend to be expensive to manufacture because they require high frequency switching components, custom-wound magnetic components, and electrical noise suppression circuitry. Moreover, because of the high frequency required to utilize reasonably sized magnetic components, electronic switching ballasts typically require electro-magnetic interference (EMI) filtering. This unavoidably adds cost and space requirements. Furthermore, robust and relatively expensive components are needed to ensure long life and efficient operation of electronic switching ballasts. In addition, a power factor correction (PFC) circuit is required to meet power factor regulations. If external light dimmers will be used, the electronic switching ballast accordingly will need extra circuitry.
Series-string current control can have very high conversion efficiency, but only for one applied voltage level. If the voltage level of the input power falls below the LED string voltage, the LEDs do not produce the required light emission. If, however, the voltage level of the input power rises above the LED string voltage, excess current flows through the LEDs and may result in damage to the LEDs. Series-string type of solid-state lighting thus requires a “ballast” resistor or active current-limiting circuitry to limit the current in the case of high input voltage. This current-limiting circuitry, nevertheless, eliminates any conversion efficiency advantage that the series-string type of solid-state lighting may have by dissipating the excess power as heat. An additional disadvantage to the series-string type of solid-state lighting used in an AC application is that the LEDs do not begin to emit light until the applied voltage approaches the string voltage. This causes a loss of cost efficiency because the LEDs are not on (i.e., emitting light) throughout the entire AC line cycle. Another disadvantage to the series-string type of solid-state lighting used in an AC application is that the string of LEDs only emits light during one half of the AC cycle, thus requiring the use of two strings of LEDs to produce light over the entire AC cycle. If light is only produced in one half of the cycle, using only one string of LEDs, the light undesirably appears to the eye to flicker at the 30 Hz half cycle frequency.